Al-(1.5%) Cu films were deposited at selected temperatures and rates into trenches on patterned wafers in order to study the deposition rate dependence of film step coverage. No substrate bias was applied to minimize resputtering of deposited material. Step coverage improves with increasing temperature and decreasing deposition rates. EVOLVE, a physically based low pressure deposition process simulator that incorporates curvature driven surface diffusion of adsorbed species, yields simulated film profiles in good agreement with experimental profiles. The results demonstrate that diffusion is a rate process critical to improving step coverage.